This invention relates to novel printing processes, particularly of the planographic type, to novel printing masters, method of forming these masters, as well as method of printing therefrom.
Conventional printing can be divided into broad process groups including relief printing, intaglio printing, and planographic printing. In relief printing, for example, the printing areas of the image carrier are raised above the plane of the substrate, which are then selectively inked for transfer to a copy sheet by direct impression. Intaglio printing involves substantially the reverse of this, in which printing areas are sunken in the image carrier, with nonprinting areas on the surface. The depressed printing areas carry applied ink which is removed in nonimage areas followed by transfer of the inked, depressed image to a copy sheet. Planographic printing is one of the better known types of printing and differs from either of the above two general types in that, printing and nonprinting areas are substantially in the same plane of the image carrier. Included within this type of printing are offset and direct lithography with the former depending on indirect image transfer from a carrier to a copy sheet, via a "blanket" or "impression" cylinder which rotates in contact with the image receiving surface and the image carrier, while the latter involves, as the term implies, direct transfer from the image carrier to the final copy or image receiving surface.
Direct lithography, while largely superseded commercially by offset lithography, has some advantages including usefulness in work where heavy ink films are essential, as well as a somewhat faster mode of operation than offset. However, because of direct contact between the image carrier and printing stock, abrasion of the image areas of lithographic plates can occur, thus life expectancy thereof is shortened, particularly if the "image" itself is relatively weak mechanically. Although it is now possible to obtain long production runs in direct lithography by means of bimetallic plates, on which the printing areas consist of one metal, and the nonprinting areas consist of a different metal, plates of this type did not exist during the time of most rapid growth of the printing industry and direct lithography, while of significant importance was therefore largely superseded by offset lithography.
In either the case of direct or offset lithography, a common denominator underlying either, is that printing and nonprinting areas are essentially in the same plane on the image carrier, and that the nonimage areas must be chemically treated to be ink repellant, and further that ink repellance in the nonimage areas must be maintained during printing by dampening the plate with a water "fountain" solution at every printing cycle. The process is thus dependent on the addition of a material such as water, which is mutually exclusive to ink, to selective areas of the imaged plate, as well as the maintenance of a balance between ink and the water during the printing process.
Planographic plate making or the formation of the imaged master, can be accomplished in a variety of ways including using a metal substrate coated with a photosensitive layer, such as a diazo compound to form a negative or positive image of a photographically applied image, as well as bimetallic plates which once imaged with a photomechanical stencil, can be selectively etched, in image or nonimage areas to provide metals of preferential sensitization for either ink or water. A more recent innovation in the formation of planographic masters, involves the use of electrophotography or xerography to image the image carrier, in which case a latent electrostatic image is formed on the surface of a photoresponsive coating which is then developed with electroscopic toner particles to form a powder image. The developed, powder image can be then transferred to a substrate and fused thereon to provide a planographic master, a solution must be applied to convert the non-imaged normally ink receptive areas of the substrate to an ink repellant or releasing condition, to thus provide a background for the relatively ink receptive, deposited toner image. After alteration of the non-image areas, the plate is then wetted with an ink which is preferentially accepted by the toner image and released by the converted hydrophilic nonimage areas.
It may, therefore, be seen that regardless of the means of imaging the planographic master, the printing system is completely dependent on the concept that a film of water which is coated over non-image areas of a printing master, will release an oleophilic or oil based ink. In this manner, the printing apparatus of the planographic variety, particularly of the offset type, necessitates the presence of various mechanical equipment for separate application of water based "fountain solution", as well as inks to the imaged master, including equipment to store these materials in adequate quantity for continuous operation, meter them as required during the process of the printing, transport them from the storage space to the printing image carrier, and distribute them properly as films to the surface of the image carrier. It may, therefore, be seen that a large amount of equipment is required to simply fulfil this function to say nothing of maintaining the delicate balance which exists between the mutually repellant ink and fountain solution thus creating numerous physical problems in metering and handling, both of which are constantly changing over the period of the printing run. Associated with this, are difficulties in maintaining proper consistency of the fountain solution, and preventing the ink from emulsifying by "backflow" of the fountain solution into the inking rollers during machine operation, as well as flowing of the fountain solution onto the offset cylinder, thus moistening the image receiving sheet causing it to curl and change dimension. Therefore, the formulation of the fountain solution referred to above, for overcoming some of these problems has become a difficult and demanding art. Planographic printing, therefore, in spite of numerous advances made therewith, is still largely dependent on operator skill in controlling the balance between ink and fountain solution, both initially and during the constantly changing conditions of the printing run. Furthermore, as opposed to complete elimination of the fountain solution, most advances in the art have been directed towards means of applying the fountain solution, or in controlling the application of it to overcome dependency on the skill of the operator to solve the attendant problems.
A different approach to overcoming the problems with fountain solutions other than the above, which is promulgated in U.S. Pat. Nos. 3,511,178; 3,677,178, 3,606,922 and 3,632,375, involves complete elimination of the need for a fountain solution or for the application of "water" to release the ink in non-image areas. This is accomplished by using a described "abhesive" background for the ink receptive image which is substantially ink releasable without regard to whether or not it is impregnated with aqueous fountain solution. The abhesive background which is releasable to the printing ink, actually keeps the ink from splitting away and transferring from the inking rollers, thus obviating a need for the fountain solution to repel the ink. This type of planographic system has therefore been characterized as a "dry" or "waterless" planographic printing system. The abhesive background in plates of this type is provided by cured silicone gums or silicone elastomers, which when dry, without being wetted by water, will not accept printing ink from an inking roller in contact therewith. On the other hand, although the need for fountain solutions has been obviated by abhesive materials of this type, nevertheless, a plate which utilizes these materials to provide ink releasable areas presents imaging problems, since the very properties of the silicone elastomer which prevents the ink from adhering to its surface also acts to prevent particulate image patterns such as toner image patterns from readily adhering to the surface thereof. Thus, the procedures normally available to sensitize lithographic masters are not suitable for planographic plates have a cured silicone elastomer coating, since various diazo sensitizers or photographic developers do not adhere well to such a surface. Therefore, the above patents overcome this problem, by constructing multilayered structures with a photosensitive layer between or overlying abhesive layers. In this manner, upon light exposure, exposed photosensitive image areas can either remain in a soluble form which is easily abraded or washed off, or be converted to an insoluble form with the non-image areas being removed. In this manner, the substrate becomes exposed in areas where removal takes place providing ink receptive areas, against the ink releasable elastomer background.
Any of the imaging systems which have heretofore been proposed with this type of planographic plate for the most part, employs photographic techniques which necessitate either additional mechanical or chemical treatment, as well as extremely long exposure times to produce an image. This not only reduces the speed involved in a complete printing operation, which includes preparation of the master, but requires the use of a planographic plate that must be carefully constructed to have a photosensitive layer, an abhesive layer and a means for securing adhesion between these two layers, in order to provide for proper imaging as well as a long life in continuous printing.
Considering the methods of imaging, available for reproduction, electrophotography and related techniques offer an advantage of simplicity, photosensitivity, and speed, which are generally unavailable to conventional photographic imaging techniques. It would therefore be highly desirable if imaging techniques of this type could be adapted to provide ink receptive image areas for abhesive, ink releasable elastomers of the type noted above. In this manner, production of printing masters could be greatly simplified, thus eliminating the need for complex photographic techniques in plate production. For example, since electrophotographic images are "developed" with toner particles, on a photoreceptive surface, this developed electrostatic image could in principle be easily adhered to an abhesive surface to provide a printing master, thus eliminating the need for any type of photosensitive layer in the abhesive coating for image formation. Such adherence, however, is difficult to accomplish since as noted above, the properties of the abhesive elastomer which prevents ink from adhering to its surface also resists adhesion of a particulate image pattern an accordingly the fixing of a toner developed electrostatic image to an elastomeric surface presents unusual problems that the ordinary image receiving surface would not.
U.S. Pat. No. 3,222,537 describes an electrophotographic element having a photoconductive layer with an overlying thermoplastic layer. A latent electrostatic image is created on the thermoplastic layer which is then developed with toner particles. Following image development, the toned record element is heated to a temperature sufficient to soften the thermoplastic layer without melting the toner particles. Under these conditions dimples are formed in the thermoplastic layer directly beneath the toner particles to form a depressed stippled image of the latent image. It is indicated that the record element may be used as a printing plate in an intaglio printing press in which the engraving ink fills the depressed image, after which the ink can be transferred to a sheet of paper. The formed element is therefore an intaglio type of master in which a depressed image is filled by ink, followed by removal of the excess ink from the surface to present ink from printing in non-image areas. The element also exclusively employs thermoplastic materials which would preferentially accept ink from a roller as contrasted to release of the ink to the roller, when a silicone elastomer is employed as the imaging surface.
The present invention is therefore directed towards a means of overcoming the inability of a surface which is substantially ink releasable, to retain a particulate image pattern in a manner which permits the use of the imaged, ink releasable member as a printing master in a continuous printing operation.
It is, therefore, an object of the instant invention to provide a novel imaged, printing master suitable for printing purposes, particularly planographic printing.
It is also an object of the instant invention to provide a method of producing such a printing master, having an ink releasable surface and useful in printing applications.
Another object is the inclusion on the printing master having an ink releasable surface, of an image which is ink receptive and capable of use in planographic type printing applications.
Still a further object of the instant invention is the provision of imaging the printing master having an ink releasable surface with a developed electrostatic image, thereby providing an imaged master which is capable of planographic reproduction.
Another object of the instant invention involves a process for printing, using a member imaged with a particulate image pattern having an ink releasable surface, and an ink receptive image which perform in a planographic type printing system without the need for an aqueous fountain solution to provide background areas of ink releasability.
Another object is therefore the provision of an offset or direct lithographic printing process which eliminates the need for an aqueous fountain solution to provide ink release in non-image areas of the master.